P/S breed is probably better than the B/B one. Scorptions are pretty generic, almsot feel kind of boring. Screech can be useful against certain pets like Death Adder or Rabbit, but Crouch is generally much better choice. Triple Snap is not my kind of ability, to much luck based. Rampage hits hard but locks you for three turns. Otherwise make use of Sting and perhaps switch away. You probably wont face many, if any, when doing PvP.

Found my rare as a secondary to a desert spider in Tanaris. They can be found wandering around as primaries, but definitely possible to find them as secondaries, especially to spiders, snakes and gold beetles. Hope this helps.

I tried what Brianne suggested, and while waiting for respawns went for a fly around, and came across a ton of rats at Writhing Mound (48,67) and got my rare stripe-tailed as a secondary there. Farming bone waste rats would seem to be the way to go :-) Thanks Brianne for the tip!

I just got a Rare right outside of Gadgetzan. But it kinda sucked cause the Gold bettle that came to fight with it was also blue! I had to only pick one lol! But hey I got him and he is Blue and so pretty =)

It seems that all joining battlepets with more than one skin can change into its other colors when summoned.

UPDATE:

The displayed skin of a skinchanger scorpid in the petjournal is grey. So if you caught a (for example) red scorpid and you have a grey one displayed in the journal, this most likely means you have a skinchanger. Summon it several times just in case. :)

An easy way to get your rare version of this pet is to go to Carrion Hill in The Bone Wastes (Terrokar Forest) - 42, 52

Rats spawn ALL OVER that hill and they have an incredibly fast respawn there. These Stripe-Tailed Scorpids often come as secondary pets with the rats, so it's very easy to battle a bunch of them and get as many retries as you need to get a rare one to pop up (all without having to leave this little hill!).